Hello,
The address book does not get a lot of use when it comes to the home setting. The address book will let you save commonly used destinations. You can set the destination to a fax number, an email address or an ifax address. This feature is usually used in an office if you are sending lots of documents/faxes to a specific location. You also have the option to register several destinations as a group. That feature is helpful if you need to send the same document to several people in a team/department/mailing list.
